The alarm, which is also tied to the fire warning system, likely malfunctioned causing the short-lived scare, Lambert said.

"We stood and waited while the heating system went through a few cycles and got nothing," Lambert said. "So there's not really anything we can do. The system is man-made and subject to malfunction."

Students and teachers were returned to the building shortly after 8 a.m. and firefighters cleared the scene by 8:30 a.m.

The call was at least the second this month in Kirksville, with KFD also temporarily closing a downtown diner after upstairs residents reported a carbon monoxide detector had gone off in early December.

There were no injuries reported in either the ECLC alarm or the downtown diner incident.

Kirksville R-III Superintendent Patrick Williams said the district's heating, ventilating and air-conditioning contractors were on campus Monday morning and examining the detector system to determine the malfunction.

Williams said the ECLC uses both water heaters, which do not pose a carbon monoxide risk, and gas-fired burners to heat the building. Anything that burns fuels can potentially pose a carbon-monoxide risk.

"In light of everything that went off Friday," Williams said, referring to the mass shooting at a school in Newtown, Conn., "and here we were evacuating a building first thing Monday morning, we wanted to be careful."

Monday's scare also came after 42 students and seven adults at a Georgia middle school were sickened by potentially lethal carbon monoxide levels on Dec. 3.

Georgia is one of many states, like Missouri, that have no requirements of carbon monoxide detectors in public school buildings.

In Kirksville, there is a municipal ordinance that requires the installation of functioning carbon monoxide detectors in residential buildings. That ordinance was enacted after seven people died from carbon monoxide exposure in 2006.

Williams noted that while there are no specific mandates for carbon monoxide detectors at the R-III, its buildings abide by strict air quality standards that require an "indoor air quality exchange" based on each building's square footage and number of occupants. A number of the buildings and specifically boiler rooms have carbon monoxide detectors installed.